tailieunhanh - excel by example a microsoft excel cookbook for electronics engineers phần 6

Chúng ta có thể nhận được thoát khỏi thanh công cụ bằng cách nhấn vào "X" ở góc bên phải trên cùng, và làm cho nó xuất hiện trở lại hoặc biến mất bằng cách kích chuột phải vào thanh công cụ và lựa chọn (hoặc bỏ chọn) tùy chọn điện trở. Tất nhiên, nó có thể xóa hoàn toàn bằng cách vào | Example 10 Voltage Regulator LM317 Figure 10-11 Inserting a button and associating a macro with it. Click on OK and right-click on the button to edit the button text to Set Voltage . Click away from the button to end the change. Clicking on the button will now run the macro. Thermal Analysis Let s add another dimension by considering the thermal implications of the design. First let s consider some theory. Heat source Figure 10-12 Analogy for thermal analysis. T junction temp. JC Tc case temp. CS Ths heat sink temp. SA Ta ambient temp. 173 Excel by Example It is possible to draw an analogy between thermal and electrical conductivity. The temperature corresponds to voltage the thermal resistance to electrical resistance and heat flow to current. Using this approach and applying it to Figure 10-12 we can write Qja Tj- Ta Pd 1 where JA is the thermal resistance from semiconductor junction to the ambient temperature in C W Tj is the junction temperature Ta is the ambient temperature and Pd is the power dissipated. 0JA Qjc cs 0SA 2 where 0JC is the thermal resistance from the junction to the case 0CS is the thermal resistance from the case to the heat sink and 0SA is the thermal resistance from the heat sink to the ambient air. The power dissipation Pd is calculated from the volt drop across the device Vd and the current flowing into it Ita. Pd Vd ỉn Vn- VJ ỉn Vm- VJ Iout 3 The last approximation is true only where the quiescent current of the device is small in comparison to the output current. In every case where heat dissipation is an issue we must first consider the total power dissipation and provide enough heat sinking that is necessary to limit the junction temperature to a safe maximum. We need to consider the worst case of an application and that may include a dead short across the output of the device. Using these generalities with our specific example of an LM317T that is the TO-220 package the absolute maximum for the junction temperature is 150 C and .